Various systems and methods to improve heat transfer between a heat transfer surface and a fluid via an engineered packing that directs the fluid to impinge the surface and thereby break down boundary layers that otherwise impede heat transfer are known. For example, apparatus employing this technique include those taught in U.S. Pat. Nos. 7,566,487, 7,976,783, and 8,257,658. These three patents disclose engineered packing structures that provide advantageous flow patterns created by flow channels that convey a fluid to and from a heat transfer surface at an oblique angle to the surface, which surface is approximately parallel to the general path of the fluid from an inlet to an outlet of a heat exchange device such as for heat transfer inside a tube or annulus or between flat plates.
Generally, the ratio of heat transfer coefficient or the Nusselt number (Nu) to pressure drop (ΔP) in a heat exchanger declines as Nu increases due to increased velocity. The above-listed patents utilize flow impingement to create high ratios of Nu/ΔP at relatively high values of Nu compared to other heat exchangers. These patents permit effective heat transfer between two or more fluids with less primary heat transfer surface area, permitting less expensive heat transfer devices.
Extended surfaces are also known as in plate and fin heat exchangers, but are only generally useful where the thermal conductivity of the extended surface material far exceeds the conductivity of the fluid transferring heat to or from the secondary surface as is the case with extended surfaces composed of copper, aluminum or noble metals and in the transfer of heat to or from a gas. Aluminum plate and fin heat exchangers enable the construction of compact and inexpensive heat exchangers for noncorrosive fluids at temperatures generally below 200° C., particularly for gases. Extended surfaces are less beneficial where the extended surfaces must be composed of carbon steel, stainless steel, nickel alloys, or other materials of relatively low thermal conductivity for corrosive or high temperature applications.